1. Field of the Invention
The invention relates generally to the field of electromagnetic surveying of the Earth's subsurface. More particularly, the invention relates to techniques for deploying electromagnetic energy sources and electromagnetic receivers for more effective subsurface surveying.
2. Background Art
Porous subsurface sedimentary rock formations are typically saturated with fluids as a result of having been deposited in a body of water during sedimentation. As a result, the fluids were initially entirely water. In some subsurface formations the water in the pore spaces has been displaced to some extent after sedimentation by hydrocarbons such as oil and gas. Thus, in some present day subsurface formations, the fluids in their pore spaces may be water, gas or oil, or mixtures of the foregoing.
Detection of formations having less than fully water-saturated pore space, that is, when oil or gas is present in the pore spaces, is of significant economic interest. Certain techniques for detection of such formations include determining existence of electrical resistivities in the subsurface that are anomalously high. The principle of such detection is based on the fact that the flow of electric current through a porous rock formation is related to the fractional volume of the pore spaces with respect to the total rock volume, the spatial configuration of the pore spaces and the electrical properties of the fluids filling the pore spaces. Brine-saturated porous rock formations, for example, are typically much less resistive than the same rock formations having hydrocarbons in some or all of the pore spaces, because brine is a relatively good electrical conductor while hydrocarbons are typically good electrical insulators.
Various techniques for measuring the electrical resistivity of subsurface rock formations are known in the art, for example, time domain electromagnetic survey techniques such as described in International Patent Application Publication No. WO 03/023452. Such techniques in general include imparting an electromagnetic field into the subsurface formations and measuring electric and/or magnetic fields induced in the subsurface formation in response to the imparted electromagnetic field. For such measurement techniques, the electromagnetic field may be imparted using an electric field transmitter, for example, by passing an electric current through a bipole electrode pair. Alternatively, a magnetic field transmitter may be used, for example, passing an electric current through a wire loop or a plurality of such loops. The receivers used to detect the responsive electromagnetic fields may be bipole electrode pairs for measuring potential differences (electric field potential), or may be wire loops, pluralities of wire loops or magnetometers for measuring magnetic field amplitude and/or the time derivatives of magnetic field amplitude. The electric current used to impart the electromagnetic field may be controlled to provide a step change in the current amplitude. Step change in the transmitter current induces what are referred to as “transient” electromagnetic fields, and the responses measured by the receivers are related to transient response of the formations in the earth's subsurface. Step change in the transmitter current may be obtained by switching the current on, switching the current off, reversing polarity, or combinations of the foregoing. A particularly advantageous form of transmitter current switching configuration used to impart the electromagnetic field is a so called “pseudo-random binary sequence” (PRBS).
In surveying an area of the subsurface using electromagnetic techniques, it is desirable to obtain signals corresponding to various distances (“offsets”) between the transmitter and receiver. In a typical survey implementation using PBRS transmitter current switching, a different bandwidth PRBS can be used for different ranges of offset. In one such example, for surveying formations below the bottom of a body of water, a receiver vessel may deploy a plurality of receivers in a selected pattern, such as a line array, on the water bottom. A separate transmitter vessel may deploy the transmitter on or at a nominal distance from the water bottom. The transmitter may be actuated and signals from the receivers recorded. Electromagnetic signals corresponding to various offsets may be obtained by moving the transmitter vessel, actuating the transmitter, and recording signals from the receivers, successively. The transmitter current is measured during actuation and the measurements thereof can then be transmitted to the receiver vessel for data quality control and processing. To survey different areas of the subsurface below the water bottom, the receiver vessel may withdraw the receivers from the water bottom, move to a different location, and once again deploy the receivers on the water bottom in a different location. The above-described transmitter deployment, transmitter actuation and signal recording may then be repeated.
Multi-transient electromagnetic (MTEM) data acquisition as described, for example in U.S. Pat. No. 6,914,433 B2 issued to Wright et al, typically uses a grounded electric transmitter and grounded electric bipole receivers, essentially as explained above with reference to marine surveying. The electrodes of the transmitter and the receivers are typically disposed in approximately the same vertical plane. The electromagnetic field induced by the transmitter may be spatially defined by the diffusion equation, and the induced field spreads out and diminishes in amplitude as it propagates through the subsurface. The measured impulse response of the Earth (e.g., the voltage impressed on any bipole receiver with respect to time) rises to a peak amplitude and then slowly decays to zero. It can be shown that the time to the response peak from any individual transient electromagnetic field event (tpeak−t0) increases as the square of the offset r, while the amplitude of the response peak decreases as the fifth power of the offset (1/r5). See, Ziolkowski, A., and D. Wright, Removal of the air wave in shallow marine transient EM data, 77th Annual International Meeting, SEG Expanded Abstracts, 26, 534-538 (2007) and Ziolkowski, A., Hobbs, B. A. and Wright, D., Multitransient electromagnetic demonstration survey in France, Geophysics, 72, 197-209 (2007).
An example acquisition system configuration for performing the method described in the Wright et al. '433 patent is shown in FIG. 1. A survey vessel 10 moves along the surface of a body of water 11 such as a lake or the ocean. The survey vessel 10 has thereon equipment shown generally at 12 and referred to for convenience as a “recording system.” A receiver cable 16 may be deployed on the water bottom 20 (or may be towed in the water), and may include a plurality of spaced apart electric bipole receivers, each shown generally as pairs of electrodes C and D. Signals resulting from voltages impressed across the receivers C, D may be communicated to a recording node 18 or similar device associated with the receiver cable 16. The vessel 10 may tow a transmitter cable 14. The transmitter cable 14 may include an electromagnetic transmitter in the form of an electric bipole, shown as spaced apart electrodes A, B. The recording system 12 may include a power supply to pass electric current across the transmitter electrodes A, B. Surveying using the arrangement shown in FIG. 1 is typically performed when the transmitter electrodes A, B are positioned longitudinally beyond one end of the receiver cable 16, in what is known as an “end-on” arrangement. Surveying is performed by passing current through the transmitter electrodes A, B and detecting response of formations 22 below the water bottom 20 by interrogating the signalinduced in the receivers C, D.
To survey specific formations (“targets”) in the subsurface, the offsets required are between two and four times the target depth in the subsurface. Deep targets therefore require relatively long offsets, however the signal amplitude is very small as a result. It has proven difficult to obtain adequate signal-to-noise ratio at long offsets and considerable effort has been devoted to optimizing signal acquisition parameters to maximize the detected signal amplitude. See for example, International Patent Application Publication No. WO 2007/104949 A1. Other techniques known in the artrelate to minimizing the noise in the detected signals. See International Patent Application Publication No. WO 2008/023174 A2.
It is desirable to have a method for electromagnetic surveying of deep targets that provides better signal amplitude.